Conoscope and method of using the same



ocr. 3o, 1945. w. l.. so@ 2,387,825

CONOSCOPE AND METHOD OF USING THE SAME ATTORNEY yPatented Oct. 30, 1945 UNITED STATES Pii'rail'rL oFFicE CONOSCOPE AND METHOD F USING f THE SAME Walter'L. Bond, Brooklyn, N. Y., assigner to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation ot New York Application May 2li, 1943, Serial No. 488,477

2 Claims.

This invention relates to a conoscope of the immersion type and to a method of using the same.

An object of the invention is to facilitate the use of the immersion type conoscope and to improve the results obtained therewith from the standpoint of accuracy. j

A further object of the invention is to facilitate the production of quartz piezoelectric crystal plates and to improve the quality thereof.

An immersion conoscope of a type in connection with which the present invention is particularly applicable is described in Patent No. 2,352,072, issued to me June 20, 1944, entitled Conoscope; this type of conoscope as described in detail in the patent referred to may comprise an immersion tank, a rotatable work supporting platform within the tank, and two optical systems which are arranged horizontally with respect to the tank and which include, respectively, a light polarizer and an analyzer. The crystal to be examined is positioned within the tank on the rotatable platform and is immersed in a fluid which has the same, or approximately the same, index of refraction as the crystal being examined. Immersion of the crystal being examined in this fluid prevents refraction, and hence bending, of the light rays as they pass through the surfaces of the crystal.

A conoscope of the type referred to may be used for making various tests on natural quartz crystals as well as on sections and partially completed piezoelectric plateswhich have been cut from the natural crystal. For example, the conoscope may be used to accurately locate the Z (optic) axis of a Z-cut piezoelectric section by a method which comprises the steps of lapping the crystal on one side as nearly as possible perpendicularly to the optic axis (which is approximately determined in advance) and sawing a section from the crystal, the cut being substantially parallel to the lapped side. The section is now placed on the platform in such a position that the convergent polarized light passestherethrough along what is assumed to be the optical axis. The position of the section is. adjusted until a system (or optical pattern) of divergent or convergent interference rings is seen through the conoscope eyepiece, the centering of this pattern indicating that the position of the crystal is such that the converged polarized light is exactly parallel to the optic axis of the crystal.

In the use of conoscopes of the nature referred to, corrections in the readings must be made under certain circumstances. In the rst place, if the refractiv'e index of the immersion fluid does not exactly match that of the crystal being examined, the light rays will be bent in passing between uid and crystal and all readl ings will be subject to correction." Secondly,

if the sectionbeing examined is not a true Z section, the ring pattern will be non-concentric and after a ring of the pattern has been centered a correction must be applied to the reading.

In accordance th a novel feature of the present invention, the refractive effect is caused to annui the ring eccentricity error by utilizing an immersion fluid the refractive index of which 4is an accurately precomputed amount away from a perfect match with the refractive index of .the quartz crystal being examined.

vention as well as appreciation ofthe various x valuable features thereof may be gained from consideration of the following detailed description andthe annexed drawing, in which:

Fig. l is a side elevational view of an immersion conoscope, a portion thereof being shown in section in order to illustrate certain parts more clearly; 1 l

Fig. 2 is an enlarged view of a portion of a quartz piezoelectric section; and

Fig. 3 is a representation of one ring of an optical pattern and the hair-lines of the conoscope'reticule.

Referring now to Fig. 1, the conoscope illustrated includes immersion tank I I, two lens tubes l and I3 arranged horizontally with respect to tank Il .and terminating in respective ports provided at diametrically opposite points in the wall thereof and three supporting legs I4, I5 and IB for supporting the conoscope assembly. Tank Il is provided with cover 2l which is hinge supported to 'permit movement between closed and open positions. Supporting leg I4 carries horizontal extension arm 22 which supports lamp housing 23.

Platform 24 is rotatably supported inside tank Il; the platform may be rotated from the exterior of tank Il by means of handwheel 25, the platform and handwheel being coupledby shaft 26. Suitable bushings and packing are provided to prevent leakage of fluid from tank Il around shaft 26. platform 24 and is rotatable therewith. The table is also slidable forward and backward on platform 24 so that the center-point of a thin plate being examined may be brought to .the intersecting point of the light beams. Transparent reference plate 4l is attached to table 21 and, in conjunction with spring clip 42, provides convenient means for supporting such small and relatively fragile objects asquartz piezoelectric secl Y tion 43 which is illustrated.

The angular position of platform 24 within the tank may be determined by observing the` relative positions of graduated dial 44 (rotatable with shaft 26) and fixed Vernier 45. Lamp 46 is provided for illumination of the dial and vez-nier.

Table 21 isremovably supported on -crosshairs" element).

4mounted between glass plates.

s summe drainage valve (not shown) may se asoman would be in accordance with the angle between the optic axis of the section and the normal to' Y the surface thereof.

curymllype, and moving toward tank II, we l first encounterlight polarizer I2 which may comprise a polarizing lm mounted between Atwo plates of glass. Polarizer 52 is mounted in carriage il; carriage 53 and polarizer 52 may be rotated, within limits, inside tuberI2 by means of lever 54.

The next item of the optical system encountered is light illter 55 which comprises three glass nlter elements properly cut and polished; in the present examplellter 55 is arranged to isolate and transmit only green light so that when the optical system is illuminated by light source 41 only green mercury light passes beyond light filter 55.

We next encounter in order concavo-convex lens 58, a pair of plano-convex lenses 51 and 1I arranged back to back, and concave-convex lens 12.

In general the functions of that portion of the .optical system located in lens `tube I2 are to polarize the light produced by source 41, to lter out all portions of the light except the green light and to cause the polarized green light to converge and to enter the crystal being examined in a cone of rays.

Considering now the portion of the optical sys'- tem housed in lens tube I3, we encounter in order concave-convex lens 13, a pair of planoconvex lenses 14 and 1I, arranged back to back, concavo-convex lens 1I and reticule 11 (the Reticule Il may comprise, for example, a glass disc having fine, straight lines scratched thereon at the desired places and illled with black pigment.' These lines may include, for example, two spaced vertical lines as shown in Fig. 3 and are used as ilducial marks during observance of the optical ring pattern produced during operationof the conoscope.

'I'he next element of the optical system is analyzer-.VII .which may comprise a polarizing film Analyzer BI is mounted in arotatable carriage and may be rotated, on occasion, Iby means of handle 82 which is coupled to the carriage by suitable gearing.

Eyepiece is positioned at the end of lens tube I3.

l The purpose, in general, of that portion of the optical system which is housed in lens tube I3 is to'correct the divergence of the light rays so that -the eye may focus them for the retina and to analyze the light rays;-l

Both sets of lenses, lilje., the set in lens tube LI2 and the set in lens tube I3, act to cause convergence of a diverging group of light rays.

It will be assumed that the conoscope is to be used to accurately locate the position of the z or optic axis of quartz piezoelectric section 43 which, it will be assumed, is a Z-cut section pre'-v pared as described above, i. e., by lapping the crystalon one side as nearly as possible perpendicularlyto the optic axis (which has been approximately determined in advance) and sawing a, section from the crystal the cut being substantially parallel to the lapped side. Section 43 is clamped against glass reference plate 4I by spring clip 42 and tank I i is iilled with` a suitable immersion iluid. 'Ihe ,reading on dial 44 after the ring 4pattern has been centered onl reticule 1l Quarts, as is well known. is a uniaxial crystal. i. e. has a single optic axis, and is birefringent. i. e. is characterized by thepower of double refraction.

Now we will assume for purposes of clearly illustrating the features oi the present invention, that two conditio xist, these conditions being commonly encountered in the use of the immersion type conoscope for the purpose described.l

The conditions to be assumed are, ilrst, that the refractive index of the fluid provided in immersion tank II does not exactly match the index oi' refraction of quartz, and, second, that section 43 is not a true Z cut section. As pointed out above, therefore, it would ordinarily be necessary to make two sets of corrections in the readings of dial 44 after the pattern had been centered, i. e., a refraction correction and a ring eccentricity correction. In accordance with the novel features of the present invention, however, one

i correction is caused, by computations now to be described, to annui the other, thereby resulting in zero" correction,

Speciiically, this is accomplished by utilizing in tank II an immersion fluid the refractive index of which differs from that of quartz (1.5462) by s. precomputed amount. I'heV proper refractive index of the fluid required to accomplish the desired object may be computed in the following manner. Referring for the moment to Fig. 2, which is an enlarged side elevational view of a portion of quartz piezoelectric section 43, the direction of the optic axis is indicated by solid line z"; a normal to the major' surfaces oi the section is indicated by solid line W and the thickness of th'e section is indicated by dimension t'. (As will appear from the formulae developed later, the method of measurement is applicable to crystals such as that illustrated in'Flg. 2 in which the-.Z axis is directed at a substantial angle away from the normal to the crystal faces.) Now at an angle of a1 from the optic axis towards the plate thickness direction the ph'ase relation N1 (,light traveling through a uniaxial crystal such as quartz breaks up into two components that travel at diil'erent velocitiesxind vibrate in respectively perpendicular planes and N1 is the phase difference developed between the fasti waves and the slow" waves in the distance t') is:

` .00917t vsincq N 1-21'm ldlBDS where a is the angle between the optic axis and normal line W,\.00917 is th'e difference between therefractive indices for the ordinaryray and the extraordinary ray for green mercury light (the type isolated vand passed by light filter l5 of Fig.` 1) traveling at right angles to the optic axis, and A is the wavelength in vacuum of the green mercury light, while at the angle oi a: from th'e optic axis away from the'plate thickness direction the phase relation Nr is:

.009l7t sinfa, os +612) radians Whence if these are equal we have:

' sin einzu, 1 cos (@-acos (-i-az) The locus of a of points fulillling the conditions expressed by Equation 1 is substantially circular, the points corresponding to phase diierences of 2f radians and integral multiples giving rise to an interference pattern consisting of a series of dark rings. `When the'angle is finite, that is, when the plate is cut at an angle other than normal to the Z axis, the pattern dened by Equation 1 consists of a set of eccentric rings. If any of these rings be selected and matched to a pair of vertical fiducial lines of reticule 11 (Fig. 1), as indicated in'Fig. 3, the center line thereof deviates from the direction of the optic axis by an angle e where elle (Z- 2 the values of ai andnz being appropriate for the selected ring.

If the separation of the reticule lines corresponds to an angle of 2d (Fig. 3) we see that The reading of dial 44 at the match of selected ring and ducial lines referred to above will be u al1-a2 Hence we have sin (d-l-e) \/cos (R-:Z (2) sin (d-e) cos (R-l-d) A Equation 2 can be given the approximate solution: e=l820 tanzd tan R minutes, or (3) e==30.3 tanzd tan R degrees (It will be observed that the correction e is independent of the thickness dimension t'.)

Considering now, for the moment, the immersion fluid which is provided in tank Il (Fig. 1), the difference between the refractive index of quartz section 43 and that of the fluid is:

H=1lqnl v (where 'nq and nz are the refractive indices of quartz and of the fluid, respectively).

Now by the law of refraction (where R is the angle of incidence and R-'u is the angle of refraction asligh't enters the quartz from the iluid) Since for small angles sin p. is closely equal to n. the deviation of a ray emerging from the quartz plate at an angle R is given by:

Comparing this with Equation 3 it will be seen that; the deviation varies with the angle of incidence in the same manner as does the eccentricity error. Consequently, the two deviations may be made to neutralize each other for any chosen ring, the correct value for H for this purpose being given by:

H :0.530 tan2d 'v ited Taking d equal to 10 degrees this would make H=.0255 from which it follows that in order to attain zero correction in accordance with the novel features of the present invention we should usein tank Il (when quartz material such as section 43 :is being examined and when d=10 degrees) ,'an immersion fluid the refractive index of which is 1.5207, that is, 1.5462 (refractive index of quartz) minus .0255.' The computed value for H would loe-different, of course, for. different values of d.

{Ihe proper immersion fluid is usually obtained by blending two different iluids one having an index of refraction higher than the desired value and the other having an index lower than the desired value. Among immersion liquids which applicant has found satisfactory for use during examination of quartz is a mixture in proper proportions of dimethyl phthalate and dichlor naphthalene.

The method of the invention is applicable not only in Vconnection with the examination of sections that are only slight departures from true Z sections but is applicable as well when the departure from a true Z section is considerable; it is believed that the `method is capable of accurate use eventhough the section departs at least as much as 60 degrees from a true Z section.

While'a specic embodiment of the invention has been selected for disclosure and detailed description, the invention is not, of course, limvin vits application to the embodiment disclosed. -The embodiment disclosed should be looked upon 'as illustrative of the invention and not as restrictive thereof;

What is claimed ist 1. In an immersion typeV conoscope for examining a quartz crystal, an immersion tank, means within said tank for supporting 'a quartz crystal being examined, means for passing rays of polarized light through said tank, a, reticule, an eyepiece for viewing said reticule, a pair of spaced, vertical guide lines on said reticule with which the optical ring pattern obtained during use of the conoscope is aligned, and an irnmerslon liquid in said tank, said liquid having an index of refraction-which is less than the refractive index of quartz by an amount equal to .530 tan2d where d'represents one-half of the angle corresponding to the separation of said vertical guide lines on said reticule.

2. In the procedure of analyzing a, birefringent crystal with respect to the location of the optic axis which includes the steps orvignnmersing the crystal in an immersion fluid contained in the tank of an immersion conoscope, passing rays of polarized light through the fluid and the crystal to produce an optical ring pattern, viewing the pattern through an eyepiece and moving the crystal to center the selected ring of the optical ring pattern with respect to the optical axis of the conoscope, the method of correcting for eccentricity of the optical ring pattern produced which comprises employing an immersion fluid in the conoscope having a refractive index which differs in magnitudeA from that of the crystal material by amamount such that theA deviation of the central ray of the selected ring of the optical ring pattern due to refraction at thecrystal face asthe ray enters the immersion iluid is equal in magnitude and opposite in direction to the deviation of said central ray within the crystal from the optic axis of said crystal.

WALTER L. BOND. 

